Electrodes for plasma arc cutting torches are typically configured with a generally cylindrical holder having a rounded or chamfered edge at the tip of the electrode and an emissive element disposed therein. The holder and the emissive element further generally combine to form a flat surface at the tip of the electrode. In this configuration, the holder is usually made of copper and has a substantially uniform wall thickness extending along the length of the holder to the tip of the electrode. During operation of the torch, the emissive element tends to erode and form a cavity inside the copper holder. Overheating and/or double arcing may then occur at the end of the copper holder due to the eroded emissive element, thus damaging the electrode and shortening the service life thereof.
A typical operational sequence of an electrode for a plasma arc cutting torch occurs as illustrated in FIG. 1. As noted above, the holder is usually made of copper and is cylindrical in shape, having a rounded or chamfered edge at the tip. A cylindrical emissive element made of, for instance, hafnium is embedded into a longitudinal bore in the holder such that the holder and the electrode are concentrically disposed with respect to each other. Together, the emissive element and the holder form a flat face at the tip of the electrode as shown in FIG. 1A. As the torch is used, the emissive element will erode and recede into the holder, as shown in FIG. 1B, thus forming a cavity within the holder. As the emissive element continues to erode from the operation of the torch and the cavity within the holder deepens, two events may possibly occur. First, as shown in FIG. 1B, double arcing may occur. That is, instead of the arc passing from point X to the workpiece, the arc will pass from point Y to the nozzle surrounding the tip of the electrode and then on to the workpiece, thereby causing damage to the electrode and/or the nozzle. Secondly, as the emissive element erodes and continues to deepen the cavity within the holder, the arc passing between the emissive element and the workpiece will overheat the holder at the tip of the electrode from which the emissive element has receded as shown in FIG. 1C. In either scenario, the holder may crack at the tip thereof, as shown in FIG. 1D, and create significant damage to the electrode and/or the surrounding nozzle. Accordingly, a number of attempts have been made to modify electrodes, consisting of a holder and an emissive element, to extend the service life thereof.
For example, U.S. Pat. No. 3,198,932 to Weatherly discloses a non-consumable electrode for use in electric arc processes such as cutting, welding, and electric arc furnace processing of metals. The '932 patent discloses an electrode that consists of a water-cooled copper holder having embedded therein an insert of zirconium. It is postulated by the patentee of the '932 patent that the operating life of the insert at relatively high currents can be increased by increasing both the diameter of the insert and the diameter of the holder while maintaining a certain dimensional relationship between the insert and the holder. Water cooling of the copper holder was also found to be critical in extending the operating life of the electrode.
In a further example, U.S. Pat. No. 4,766,349 to Johansson et al. discloses an electrode for electric arc processes composed of a water-cooled holder into which is fitted a case-hardened diffusion-coated insert of zirconium or hafnium, wherein the diffusion zone consists of carbide, nitride, boride, or silicide. The compounds in the diffusion zone have very high melting points which suppress reactions between the holder and the insert that cause deterioration of the electrode. However, the introduction of the diffusion-coated insert into the water-cooled copper holder must be accompanied with a protecting finish of nickel, chromium, or platinum metal on the surface of the holder in order to prevent its deterioration during operation.
In addition, U.S. Pat. No. 3,930,139 to Bykhovsky et al. discloses a non-consumable electrode for oxygen arc working comprising a holder produced from copper or alloys thereof and an active insert fastened to the end face of the holder. The insert is in thermal and electrical contact with the holder through a metal distance piece disposed between the insert and the holder and over their entire contact surface area. The metal distance piece is manufactured from aluminum or alloys thereof and the insert is made from hafnium. In operation of the torch, the insert is still subject to erosion. However, when operating in oxygen, an aluminum oxide is formed on the metal spacer. The aluminum oxide is a high melting temperature compound which acts as a thermal shield protecting the copper holder both from overheating and oxidation.
Thus, attempts to extend the service life of electrodes for plasma arc torches generally involve increasing the size of both the holder and the insert, as disclosed in the '932 patent to Weatherly, or providing a barrier between the insert and the holder, such as the diffusion zone disclosed in the '349 patent to Johansson et al., and the metal distance piece disclosed in the '139 patent to Bykhovsky et al. Increasing the size of both the insert and the holder in a specified dimensional relationship results in a larger electrode which may be cumbersome and/or unsuitable for precision work. In addition, special diffusion treatments for the insert may be difficult to manufacture consistently and/or may not be cost effective in relation to the gain in the life of the electrode. Further, the addition of a distance piece between the insert and the holder increases the number of components in the assembly and may also add to the cost and increase the difficulty of assembly of the electrode.
Thus, there exists a need for a simple, cost-effective electrode for a plasma arc cutting torch having a suitably long service life. Preferably, the electrode comprises a holder having an emissive element, wherein the holder and the emissive insert are made of materials with suitable characteristics. In addition, there exists a need for an electrode for a plasma arc cutting torch which avoids the problems of double arcing or overheating as the emissive element erodes within the holder.